You can't change anything in a listing once it already has bids.

For future reference if you want to list an item as 'Pick Up Only' you have to click on 'Create Postage Policy' and a new window will open  - give the policy a name (anything you want such as 'Pick Up Policy') -  from the drop down Domestic Postage list select 'No Postage - Pick Up Only' - then Save and complete the listing.